88 350 TBI cold start stumble.

[Supercharged111]

I think it was 16 this morning when I started it.

May well be the same thing mine pulled. I think the early TBIs are a bit more rudimentary than the later ones.

 

[PlayingWithTBI]

I think it was 16 this morning when I started it.

I can't help you there, my truck has never seen 16* unless it was shipped from the factory in winter

 

[Schurkey]

I changed about a year ago. Think I should try another one?

I'm re-thinking that. The O2 sensor is probably not warm enough during fast-idle to provide a stable signal. And if the misfire is in the right bank, the sensor might not pick it up at all.

Even so, there should be SOME indication of a misfire on the scan tool--instantaneous drop in manifold vacuum, slight drop in RPM...something.

Any indications on the spark plugs?

 

[Steven Petersen]

There’s probably a 50 rpm drop. It’s just a quick miss or stumble. Does it usually 1-5 times during the first 30 seconds or so when it’s at the fastest idle. I haven’t looked at any plugs yet. They are new AC plugs that I put in about 1,000 miles ago. Wouldn’t a fouled plug act up at other times besides fast idle?

 

[Schurkey]

If you hold the "fast idle" speed once it's warmed-up, does it still stumble?

I wasn't thinking the plugs were fouled, but there might be indication of misfire if you pulled them soon after hearing the misfire.

 

[Steven Petersen]

If you hold the "fast idle" speed once it's warmed-up, does it still stumble?

I wasn't thinking the plugs were fouled, but there might be indication of misfire if you pulled them soon after hearing the misfire.

No, it will never stumble after it’s warmed past probably 45 degrees coolant temperature. I’ve held a fast idle trying to check that before. There’s definitely a correlation between low ambient temp, low coolant temp, and the stumble.

 

[Schurkey]

Coolant temp sensor reads correct on the scan tool before you fire up the engine? (i.e., it's not inaccurate?)

You're using fresh (winter-blended) gasoline?

 

[Steven Petersen]

Coolant temp sensor reads correct on the scan tool before you fire up the engine? (i.e., it's not inaccurate?)

You're using fresh (winter-blended) gasoline?

It usually shows 34-35 before start up. This is my first time owning a scanner but I assume this is normal? It’s well below freezing ambient everytime I’ve warmed up using the scanner.

Gas I have no clue other than it’s fresh and I get it from a station in Bonners Ferry, Idaho. And we have 2 other vehicles (2014 Sierra and 2018 Yukon) that get gas from the same station with no issues.

 

[Schurkey]

It usually shows 34-35 before start up. This is my first time owning a scanner but I assume this is normal? It’s well below freezing ambient everytime I’ve warmed up using the scanner..

You're saying the coolant sensor is showing the temperature of the engine is warmer than actual temperature, before you start the engine?

 

[Supercharged111]

I'm re-thinking that. The O2 sensor is probably not warm enough during fast-idle to provide a stable signal. And if the misfire is in the right bank, the sensor might not pick it up at all.

Even so, there should be SOME indication of a misfire on the scan tool--instantaneous drop in manifold vacuum, slight drop in RPM...something.

Any indications on the spark plugs?

In 88 there was no O2 heater. I retrofitted a universal heated sensor to my 88 briefly before I sold it.